Dense, long-term, wide-area deployments of tiny wireless sensors can potentially enable many novel and useful monitoring-based applications. A fundamental challenge in realizing this potential is to supply each sensor with enough power for its sensing and communication needs. One possible solution is to structure the sensors as wireless transceivers (or wisps), based on radio-frequency-identification tags, that meet all their power needs by scavenging off the interrogating signal from ambient high-power RFID readers. A usage model for wisps enables a variety of applications related to detecting day-to-day human activities. Evaluation of an experimental 1-bit accelerometer wisp show that it performs, for the most part, in the range required by the applications.